[Chap. XXXIX MUTATIONS 475 



as described in the preceding chapter. Among the characters that appear 

 to be results of changes in single genes are red-flowered snapdragons 

 and fuzzless peaches, or nectarines. The relationship is not so easily de- 

 tected when the development of the character depends specifically upon 

 the interaction of several genes. 



We should not be misled into attributing the development of a par- 

 ticular character solely to the influence of a single gene, even though it 

 appears, or fails to appear, when the particular gene is present or absent. 

 The total development of a plant, or of any part of it, is dependent upon 

 the collective influences of numerous genes. When a plant having all of 

 its genes is compared with a plant in which one gene is lacking, we may 

 be able to detect one or more effects of this one gene on the development 

 of the plant. The number of different kinds of influences a particular 

 gene may have has never been fully discovered. 



Thus far we have emphasized the orderliness of the numerous proc- 

 esses that occur every time cells divide, and when plants reproduce 

 sexually. It is time now to turn our attention to some of the irregularities, 

 or aberrations, that sometimes occur, and to try to visualize their rela- 

 tion to the changes that are occurring in plants today and have been 

 occurring in plants for a billion years or more. 



Mutations in the vegetative cells of plants. We have seen that the exact 

 duplication of the hereditary mechanism when a cell divides in the 

 growing regions of the vegetative body of a plant depends upon an 

 exact duplication of all its chromosomes and genes, and also upon a very 

 definite distribution and assemblage of these daughter chromosomes in 

 the nuclei of the two new cells. 



When we are able to visualize the usual orderly procedure of events 

 in cell division, we can begin to appreciate some of the irregularities 

 which occur occasionally, and with a little scientific imagination we can 

 even predict what some of them will be. For example, the chromosomes 

 may divide without further division of the nucleus or of the cell, resulting 

 in a cell with a tetraploid (4n) number of chromosomes. Some or all of 

 the chromosomes on their way to the poles of a cell might divide a 

 second time before a new cell wall is formed, resulting in cells with an 

 increase in number of chromosomes. Some of the chromosomes might 

 fail to divide, and one of the new cells would lack one or more chromo- 

 somes. The two halves of a divided chromosome might go to the same 

 pole of the spindle, resulting in two new cells, one of which has 2n + 

 1, the other 2n — 1 chromosomes. A chromosome might become 



